I apologize for the delay - I meant to post my "final" recommendation regarding Civ IV last Wednesday. Various preparations and schemes prevented that from happening in a timely fashion.
So, here we go. Should you buy Civilization IV.
My answer is "Yes", with the two following caveats:
1. You have bought a desktop computer within the last 2 years OR a good laptop within the past year OR are willing to spend a little extra money on upgrades if the game doesn't run well out of the box;
2. You have played and enjoyed at least one of the first three canonical Civilization games.
My personal bottom line is, this is the best Civ yet. I never really paid attention to previews that talked about the "streamlined interface," because the old interface never bothered me; it wasn't until I started playing that I appreciated the magnitude of what they've done. It's been reminiscent of my experience playing Civ 2, when I realized that they had solved problems which I hadn't realized existed. The pace of the game is much quicker now from turn to turn, because you aren't flipping through a dozen city screens in the late game, and as a result you can actually keep track of your strategy and the big picture you're trying to accomplish. There are more times than I care to admit when I would spend 10-15 minutes on a single turn in Civ 2, keeping my cities happy and moving units, only to stare at a transport in the next turn wondering where I meant to take it.
For more information, look at my earlier posts. The new features all work. (I still can't speak for the multiplayer.) There are already some really exciting things going on with mods, which of course kept Civ 2 going for a long while. They've removed the specific things that made Civ 3 occasionally infuriating and brought back the fun empire-building of earlier games. (That said, if all you know is Civ 3, this will feel familiar and even better to you.)
Again, my earlier posts contain quibbles, some of which will probably be fixed in patches (stuttering wonder movies and the civilopedia), others of which will probably stay (modern units that fail to dominate over older units), but the only thing that is likely to cause real grief is if you spend $50 on this game and then can't play it on your computer.
And what if you haven't played Civilization before? Well, I'm surprised you're reading this blog for one; virtually everyone I know has gotten sucked in at one time or another. Civilization is, simply put, the best turn-based strategy game ever, so you'll like it if you've enjoyed other games in the genre like Master of Orion, X-COM, or Worms. And if you don't know turn-based strategy games? Think board games, basically. You have a board and you move pieces around on it, competing against others to reach your goals. It's more complex than that, of course, but in a way it's just a very advanced version of Chess or Monopoly. The best thing you can do is find another reader of this blog - because I can guarantee they will have played Civilization. Get them to buy the game, then watch them play for a little while, then play some yourself on Settler or Chieftan mode. If you're able to crush a rival civ or start building spaceships, you'll feel an irrational boost of self-esteem and be hooked as well.
I'm rambling now. (As opposed to normal, when I run an extremely tight and focused blog.) Once again: if you've liked a previous Civ game and have a decent computer, buy this game. You will not be disappointed.